1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas which is used in exhaust systems of automobiles. More particularly, it relates to an improvement on heat resistance of an exhaust-gas-purifying catalyst which includes a ceria-zirconia composite oxide.
2. Description of the Related Art
As an exhaust-gas-purifying catalyst which is used in exhaust systems of automobiles, a 3-way catalyst is used widely which purifies HC, CO and NOx simultaneously. This 3-way catalyst is arranged so that a porous oxide, such as alumina (Al2O3) and silica (SiO2), makes a support and a noble metal, such as platinum (Pt) and rhodium (Rh), is loaded on the support. The HC and CO adsorbed on the catalyst is purified by oxidizing, and the NOx is purified by reducing.
This 3-way catalyst is designed so that it exhibits the highest conversions when it contacts with an exhaust gas whose air-fuel ratio of the air-fuel mixture is an ideal air-fuel ratio, i.e., a stoichiometric atmosphere. However, depending on the driving conditions of automobiles, the actual air-fuel ratio fluctuates on a rich side or a lean side about the stoichiometric atmosphere. Accordingly, the exhaust-gas atmosphere fluctuates as well. Consequently, high purifying performance cannot necessarily be secured by the 3-way catalyst of the aforementioned arrangement alone.
Hence, a 3-way catalyst has been known in which ceria (CeO2) is included in the support. Since the ceria has an oxygen storage-and-release ability, oxygen is stored in the ceria in oxygen rich atmospheres, and is released in oxygen lean atmospheres. Therefore, the fluctuation of the exhaust-gas atmospheres can be relieved by including the ceria, and the purifying performance is upgraded. Further, it is preferable to include a ceria-zirconia composite oxide, in which ceria is composited with zirconia (ZrO2) in a support. By compositing with the zirconia, the thermal stability of the ceria is enhanced remarkably.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 10-182,155 discloses an exhaust-gas-purifying catalyst in which a noble metal is loaded on a composite oxide which includes at least two members selected from the group consisting of cerium, zirconium and aluminum.
By the way, the recent increment of high-speed driving, etc., resulted in considerably high temperatures of exhaust gases of automobiles. Accordingly, it is necessary to carry out the durability test on exhaust-gas-purifying catalysts under severer conditions.
In the aforementioned conventional exhaust-gas-purifying catalyst, no problem arises when the catalyst is subjected to a durability test in which it is heated at 1,000xc2x0 C. for a few hours, for instance. However, when it is subjected to a severe durability in which it is heated at 1,000xc2x0 C. for 20 hours, the following have been found out. Namely, depending on the composition of the support, the phase separation occurs in the solid solution of ceria and zirconia.
Further, general exhaust-gas-purifying catalysts employ a honeycomb-shaped substrate, which is formed of cordierite or a metallic foil and exhibits good heat resistance, and are used by forming a coating layer, which is formed on the substrate and includes a loaded particle loaded with a noble metal. However, when a catalyst, which is provided with a conventional coating layer including a composite oxide support disclosed in Japanese Unexamined Patent Publication No. 10-182,155, is subjected to a severe durability test, the following have been found out. Namely, there arises a drawback in that the coating layer is cracked by the thermal contraction of the support, or in that the coating layer comes off.
It is therefore an object of the present invention to further improve the heat resistance of an exhaust-gas-purifying catalyst, in which a noble metal is loaded on a support including a ceria-zirconia solid solution, to make the catalyst exhibit high purifying activity even after a severe durability test, and to inhibit its coating layer from cracking and coming off.
In a first aspect of the present invention, a catalyst for purifying an exhaust gas comprises:
a support including a mixture containing a porous oxide and a composite oxide; and
a noble metal loaded on the support,
the composite oxide expressed by a general formula (1);
(Al2O3)a(CeO2)b(ZrO2)1xe2x88x92bxe2x80x83xe2x80x83(1)
xe2x80x83in which the values xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are molar ratios falling in the range of 0.4xe2x89xa6xe2x80x9caxe2x80x9dxe2x89xa62.5, and 0.2xe2x89xa6xe2x80x9cbxe2x80x9dxe2x89xa60.7, respectively; and
the support including a particle having a particle diameter of 5 xcexcm or more in an amount of 30% by volume or more.
In a second aspect of the present invention, a catalyst for purifying an exhaust gas comprises:
a support including a mixture containing a porous oxide and a composite oxide; and
a noble metal loaded on the support;
the composite oxide expressed by a general formula (2);
(Al2O3)a(CeO2)b(ZrO2)1xe2x88x92b(Y2O3)cxe2x80x83xe2x80x83(2)
xe2x80x83in which xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, and xe2x80x9ccxe2x80x9d are molar ratios falling in the range of 0.4xe2x89xa6xe2x80x9caxe2x80x9dxe2x89xa62.5, 0.2xe2x89xa6xe2x80x9cbxe2x80x9dxe2x89xa60.7, and 0.01xe2x89xa6xe2x80x9ccxe2x80x9dxe2x89xa60.2, respectively; and
the support including a particle having a particle diameter of 5 xcexcm or more in an amount of 30% by volume or more.
In a third aspect of the present invention, a catalyst for purifying an exhaust gas comprises:
a support including a mixture containing a porous oxide and a composite oxide; and
a noble metal loaded on the support;
the composite oxide expressed by a general formula (3);
(Al2O3)a(CeO2)b(ZrO2)1xe2x88x92b(La2O3)dxe2x80x83xe2x80x83(3)
xe2x80x83in which xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, and xe2x80x9cdxe2x80x9d are molar ratios falling in the range of 0.4xe2x89xa6xe2x80x9caxe2x80x9dxe2x89xa62.5, 0.2xe2x89xa6xe2x80x9cbxe2x80x9dxe2x89xa60.7, and 0.005xe2x89xa6xe2x80x9cdxe2x80x9dxe2x89xa60.1, respectively; and the support including a particle having a particle diameter of 5 xcexcm or more in an amount of 30% by volume or more.
In a fourth aspect of the present invention, a catalyst for purifying an exhaust gas comprises:
a support including a mixture containing a porous oxide and a composite oxide; and
a noble metal loaded on the support;
the composite oxide expressed by a general formula (4);
(Al2O3)a(CeO2)b(ZrO2)1xe2x88x92b(Y2O3)c(La2O3)dxe2x80x83xe2x80x83(4)
in which the values xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9ccxe2x80x9d and xe2x80x9cdxe2x80x9d are molar ratios falling in the range of 0.4xe2x89xa6xe2x80x9caxe2x80x9dxe2x89xa62.5, 0.2xe2x89xa6xe2x80x9cbxe2x80x9dxe2x89xa60.7, 0.01xe2x89xa6xe2x80x9ccxe2x80x9dxe2x89xa60.2, and 0.005xe2x89xa6xe2x80x9cdxe2x80x9dxe2x89xa60.1, respectively; and
the support including a particle having a particle diameter of 5 xcexcm or more in an amount of 30% by volume or more.
In a fifth aspect of the present invention, the CeO2 and the ZrO2 contained in the composite oxide, form a solid solution; and a solubility of the ZrO2 is 50% or more in the composite oxide.
In a sixth aspect of the present invention, a mean crystal diameter of the ceria-zirconia solid solution, contained in the composite oxide, is 10 nm or less by an X-ray diffraction measurement after heating the support at 1,000xc2x0 C. for 5 hours or more.
In a seventh aspect of the present invention, a weight ratio of the porous oxide and the composite oxide in the support satisfies the condition 0.30xe2x89xa6the composite oxide/(the porous oxide+the composite oxide)xe2x89xa60.7.
In a eighth aspect of the present invention, Pd is loaded on the composite oxide; and at least one of Pt and Rh is loaded on the support.
In a ninth aspect of the present invention, the support includes a particle having a particle diameter of 5 xcexcm or more in an amount of less than 85% by volume.
According to the present invention, the coating layer of the present exhaust-gas-purifying catalyst hardly cracks and comes off after a severe durability test. In the coating layer, the ceria-zirconia composite oxide exhibits a high specific surface area, and is kept to be highly dispersed. Thus, the present exhaust-gas-purifying catalyst is good in terms of the durability.